The present invention relates to a method for determining the residual oil saturation of a hydrocarbon-producing formation penetrated by a borehole. In particular, the method pertains to a method for determining the residual oil saturation of a petroleum producing reservoir which has been reduced to a residual oil saturation by water displacement -- either by natural or induced means. At the present time there is considerable interest in tertiary recovery methods for producing additional crude oil from old reservoirs. Several methods of tertiary recovery have been proposed. They are expensive to apply and whether they are economic depends to a large extent upon the oil remaining in place in the reservoir. In situations where it is feasible to drill a new borehole, various logging devices may be used to determine the residual oil remaining in the formation. Often, the economics of the project will not justify drilling a new borehole (or boreholes) for the sole purpose of determining the residual oil in place. Thus, measurements for determining the residual oil must be made using a previously drilled borehole that is cased with steel tubular members. The presence of steel casing in the borehole limits the types of logging devices that can be used to those that have the capability of logging a formation through steel casing. The devices are either pulsed neutron capture types or various other types of logging devices which measure the induced gamma of neutron activity of the formation.
Several patents have been issued which disclose what is known as a log-inject-log method using pulsed neutron capture logs to determine the residual oil present in a reservoir. For example, U.S. Pat. No. 3,562,523 describes a method for determining residual oil using a pulsed neutron capture tool in which the formation is first logged with the residual oil and water in place, the water phase is then replaced by water having a different neutron capture cross-section and the formation is again logged. By appropriate calculations the residual oil saturation may be deduced. Another U.S. Pat. No. 3,631,245 describes a similar method, but instead of the water being replaced, the oil is removed from the formation by using various chemical flooding or other methods. While one or both of these methods have been used for determining residual oil saturation, they are practiced only with difficulty due to the fact that both require the use of extremely sophisticated borehole devices and data treatment. In addition, knowledge as to thermal neutron diffusion effects is necessary and compensation in some manner must be made as pointed out in U.S. Pat. No. 3,852,593. Furthermore, the methods require either a knowledge of the porosity of the formation or the neutron capture cross-section of the residual oil. These parameters can be very difficult or impossible to obtain with the accuracy required in this type of application. The proposed method circumvents all of these deficiencies.
Another U.S. Pat. No. 3,894,584 describes a method for determining residual oil in which the natural radioactivity of the formation is measured followed by injection into the formation of an aqueous solution containing radioactive tracer elements. The formation is again logged and the oil removed from the formation by a chemical flooding or other method. After the oil is removed from the formation, the chemical flooding agent is removed by injecting the original formation water or brine in the zone and the formation again logged. From the three logs plus the porosity of the formation one can presumably determine the residual oil saturation of the formation. While this method may produce satisfactory results, it does contain unnecessary steps and in addition, fails to solve other problems. For example, the method requires three separate logging runs in the formation and requires that one know the porosity of the formation quite accurately in order to calculate the residual oil saturation accurately. Further, since the method makes no provisions for compensating for the radioactivity of the borehole fluid, there is an additional serious question concerning the accuracy of the results obtainable.